1. Field of the Invention
The present invention relates to secondary (rechargeable) alkali-ion batteries. More specifically, the invention relates to materials for use as electrodes for an alkali-ion battery. The invention provides transition-metal compounds having the ordered olivine or the rhombohedral NASICON structure and containing the polyanion (PO.sub.4).sup.3- as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.
2. Description of the Related Art
Present-day lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li.sup.+ ions to the Li.sup.+ -ion electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li.sup.+ ions are inserted reversibly from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible. On charge, removal of electrons from the cathode by an external field releases Li.sup.+ ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li.sup.+ ions back into the anode to restore it to its original composition.
Present-day rechargeable lithium-ion batteries use a coke material into which lithium is inserted reversibly as the anode and a layered or framework transition-metal oxide is used as the cathode host material (Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/or Ni are expensive and may degrade due to the incorporation of unwanted species from the electrolyte. Oxides such as Li.sub.1.+-.x Mn.sub.2 !O.sub.4, which has the M.sub.2 !O.sub.4 spinel framework, provide strong bonding in three dimensions and an interconnected interstitial space for lithium insertion. However, the small size of the O.sup.2- ion restricts the free volume available to the Li.sup.+ ions, which limits the power capability of the electrodes. Although substitution of a larger S.sup.2- ion for the O.sup.2- ion increases the free volume available to the Li.sup.+ ions, it also reduces the output voltage of an elementary cell.
A host material that will provide a larger free volume for Li.sup.+ -ion motion in the interstitial space would allow realization of a higher lithium-ion conductivity .sigma..sub.Li, and hence higher power densities. An oxide is needed for output voltage, and hence higher energy density. An inexpensive, non-polluting transition-metal atom would make the battery environmentally benign.